RF circuitry may not work well at low frequency because RF transistors which are built with GaAs or SiGe usually distort low-frequency components in a broadband signal by increasing the level of such low-frequency components with respect to the level of high-frequency components in the broadband signal. Some devices like PIN diodes cannot operate at frequencies below 1 MHz at all because of limited carrier lifetime.
On the other hand, many applications require the electronic elements in a circuit to have the ability to work with broadband signals, often including low-frequency and DC signals. For example, high-bandwidth low-pass filters, amplifiers and filters for NRZ codes as used in disk drives and other magnetic recording devices, as well as amplifiers and filters for the perpendicular recording signals in disk drives and other magnetic recording devices all have to process the broadband signals, which contain a significant amount of low-frequency components, including DC signals.
Therefore, there is a need for a new approach to process broadband signals with limited distortion in low-frequency components of the broadband signals.